Several types of equipment used downhole in oil and gas wells, or beneath the surface of the sea adjacent to offshore wells, are electrically operated or actuated. Examples of such equipment include certain wireline tools and remote well telemetry equipment. The electrical power required can be provided by connecting the device to a surface power source via electrical cables, or by placing a power source near the site of the device itself. Often it is not practical to use electrical cables running from the surface to the subterranean or subsea site of the electrically-powered device, because of the great distance involved, or because the cables can interfere with the passage of other equipment through the wellbore, and are vulnerable to being damaged during well operations.
Batteries can be used as a local source of power for downhole and subsea electrical devices, but are subject to their own problems. For example, increasing the power and energy generation capacity of a battery generally requires a proportionate increase in the size of the battery, which can present difficulties given the space constraints that exist in wellbores. Also, batteries will typically need to be electrically recharged at some point, thereby often making it necessary to provide some type of recharging equipment in physical proximity to the battery.
Fuel cells make use of an electrochemical reaction involving a fuel and an oxidant in a cell that comprises an anode, cathode, and electrolyte, to generate electricity without also generating the unwanted by-products associated with combustion, while providing relatively higher energy efficiency. Thus, fuel cells potentially have a number of advantages over other power generation or storage means in many applications. The fuel cells of the present invention can be used in a variety of applications. Although the invention is primarily described herein in relation to applications involving subterranean wellbores, it should be understood that the invention can be used in applications other than wellbore applications.
A number of obstacles have hindered the use of fuel cells in downhole and subsea applications. For instance, fuel cells typically include one or more pumps to provide circulation of fuel and/or oxidant in a closed loop through the cell. If such a pump fails downhole, repair or replacement can be extremely expensive, given the need to retrieve the fuel cell to the surface. Further, the operation of the pumps consumes some of the energy produced by the cell, thus reducing the net power yield available to operate an external device. This latter point can be a significant problem in downhole or subsea applications in which a supply of power is needed for an extended period of time, and yet space constraints limit the ability to simply increase the size of the fuel and oxidant reservoirs. Additionally, the reaction product, water vapor, needs to be removed from the fuel cell stack in order to continuously run the fuel cell. Removal of the water downhole and in a subsea environment presents a challenge because the surrounding pressure is higher than that present in a conventional fuel cell placed at surface in an ambient environment and operating in air. Using a pump to expel the water into the high pressure downhole or subsea environment may require a large amount of power making such a system impractical.
VanBerg U.S. Pat. No. 5,202,194 describes a power supply for providing electricity to electrical circuits located downhole in a well. The power supply comprises a fuel cell, which is fed by hydrogen from a pressure container and oxygen from compressed oxygen gas bottles. Pressure regulators are located in the line between the hydrogen container and the fuel cell, and in the line between the oxygen bottles and the fuel cell. A pump is used to eject water from the fuel cell into the wellbore. The need to have a water outflow path from the interior of this fuel cell to its exterior raises potential reliability issues and may be impractical for downhole use.
There is a need for a new fuel cell operation concept and an improved fuel cell apparatus that can provide the electrical power needed to operate various downhole and subsea equipment.